Investments (Narrative) (Details) $ in Millions	1 Months Ended	6 Months Ended
Jul. 31, 2017 USD ($) MW	Jun. 30, 2016 USD ($)
Schedule of Investments [Line Items]
Proceeds from liquidation of company-owned life insurance policies		$ 31
IPL [Member]
Schedule of Investments [Line Items]
Proceeds from liquidation of company-owned life insurance policies		$ 19
Subsequent Event [Member] \| Alliant Energy Finance LLC [Member]
Schedule of Investments [Line Items]
Cash equity ownership interest, ownership percentage	50.00%
Electric capacity of non-regulated wind farm \| MW	225
Term Loan Credit Agreement [Member] \| Subsequent Event [Member] \| Alliant Energy Finance LLC [Member]
Schedule of Investments [Line Items]
Variable-rate term loan credit agreement	$ 95
Variable-rate term loan credit agreement, period	364 days
Scenario, Forecast [Member] \| Alliant Energy Finance LLC [Member]
Schedule of Investments [Line Items]
Increase in assets from acquisition, subject to working capital adjustments	$ 98

Line items represent reporting concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Electric Capacity Of Wind Project

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The net cash outflow or inflow for net proceeds and payments derived from carrying life insurance policies for which the entity is the beneficiary.

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